1. Field of the Invention
Aspects of the present invention relate to an image forming apparatus, and more particularly, to a fixing unit to fix an image transferred to a printing medium and an image forming apparatus having the fixing unit.
2. Description of the Related Art
In general, image forming apparatuses, such as copiers, printers, facsimile machines, and multi-function machines embodying the functions of the above-mentioned devices in a single device, comprise photosensitive members on which electrostatic latent images are formed, developing units to develop the electrostatic latent images, transferring units to transfer the developed images onto printing media, and fixing units to fix the transferred images onto the printing media. FIG. 1 shows an example of a fixing unit.
The fixing unit of a conventional image forming apparatus illustrated in FIG. 1 includes a heating roller 1 and a pressing roller 2 which rotate and contact with each other. A heat source 1a is mounted within the heating roller 1, and the pressing roller 2 is biased toward the heating roller 1 by a pressing spring (not illustrated). In the heating and pressing rollers 1 and 2, an elastic rubber layer 1c and a release layer 2c are laminated on outer surfaces of metal pipes 1b and 2b, respectively.
In the configuration described above, a printing medium P to which an image is transferred passes through a fixing nip N between the heating and pressing rollers 1 and 2, and accordingly, heat and pressure are applied to the image on the printing medium P such that the image is fixed to the printing medium.
In order to decrease fixing time to achieve high-speed printing, outer diameters of the heating and pressing rollers 1 and 2 may be expanded or the thickness of the elastic rubber layer may be thickened. Accordingly, there is provided a method for shortening the fixing time by increasing the size of the fixing nip N.
However, if the outer diameters of the heating and pressing rollers 1 and 2 are increased, the entire volume of the image forming apparatus will also increase as well as the warm-up time, causing an increase in cost. Additionally, if the elastic rubber layer is thickened, the warm-up time may further increase, the fixing efficiency may deteriorate, and the durability may be reduced due to a concomitant increase in the fixing temperature.
Fixing efficiency may be enhanced by increasing the pressurizing force of the pressing roller 2, but other problems arise, such as distortion of the elastic rubber layer, a decrease in durability, jamming caused by a decrease in the transferring force of the printing medium P, and a necessary increase in driving torque.